Method for treating magnesium articles



\./ Feb. 26, 1935. J. A. GANN Er AL 1,992,205

METHOD FOR TREATING MAGNESIUM ARTICLES Filed Dec. 25, 1931 B lo 9; 2

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invention deals broadly with the cleaning of Patented Feb. 26, 1935UNITED STATES METHOD FOR TREATING MAGNESIUM ARTICLES John A. Gann andWilliam H. Gross, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Michigan Application December 23, 1931, Serial No.582,786

5 Claims.

The present invention relates to a method and composition for treatingor cleaning the surfaces of magnesium and magnesium alloy articles toremove surface contaminations and impurities While the magnesium andmagnesium alloy articles, it is especially well adapted for cleaningforeign matter from portions thereof that have been Welded by means of agas flame or an electric arc.

It has been heretofore proposed (British Pat. 287,450) to improve thecorrosion resistance of magesium articles by treating with an aqueoussolution of nitric acid and potassium bichromate, the use of sulphuricacid bichromate, or hydrochloric acid -lbichromate solutions for thispurpose being expressly avoided. We are also aware that an aqueoussolution containing to 30 per cent of sulphuric acid has been proposedin U. S. Patent 1,795,473 for use in cleaning magnesium and magnesiumalloy articles. The use of sulphuric acid in this manner, we have found,is satisfactory for certain purposes Where the loss of metal, due to theattack of the acid, is not objectionable. For many purposes, however,such as the cleaning of magnesium and magnesium alloy sheets, welds, andthe like, we have found that the action of 10 to 30 per cent aqueoussulphuric acid alone, is, in general, too drastic. In this connection wehave found that the addition of a substantial amount of a magnesium saltto an aqueous sulphuric acid solution has a decided beneficial effect inreducing the attack of the sulphuric acid on the magnesium as animprovement over the procedure described in the aforementioned U. S.Patent 1,795,473.

Accordingly, among the objects of the present invention is the provisionof a method and composition for use in cleaning magnesium and magnesiumalloy articles without undue attack on the metal itself. Other objectsand advantages will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, consists of the methods, steps, and compositions hereinafter fullydescribed and particularly pointed out in the claims, the annexeddrawing and the following description setting forth in detail severalforms of composition and modes of carrying out the invention, suchdisclosed forms and modes illustrating, however, but several of thevarious ways in which the principle of the invention may be used.

In said annexed drawing:-

Fig. 1 is a series of curves illustrating in typical manner thereduction in attack of sulphuric acid on magnesium articles when ourimproved agents are used; Fig. 2 is a curve showing the reduction inattack of sulphuric acid on magnesium articles when increasing amountsof magnesium sulphate are added to the solution.

Magnesium and magnesium alloy articles, such as ingots, castings,wrought shapes, sheet metal, welded sections and the like, beforethorough cleaning, may have surface contaminations and impurities whichact in exposure to air and moisture to cause the article to corrode, andwhere the impurities are present in substantial amount, such as weldingflux and oxides on and around a weld, the action is much more severe.The attack caused by such surface contaminations and impurities, We havefound, can be eliminated without undue attack on the metal article beingcleaned, by treating same in an aqueous bath of sulphuric acidcontaining a suitable magnesium salt with or without the addition of asuitable chromium compound. Magnesium sulphate, magnesium nitrate,magnesium chloride, and magnesium phosphate, which are salts of strongacids and are soluble in the solution under oonsideration, are typicalexamples of suitable magnesium salts; and chromic acid, or salts thereofincluding chromates or bichromates of magnesium, calcium, ammonium, andalkali metals, are typical examples of suitable chromium compounds. Inits broadest phase, the present invention contemplates the use ofsuitable magnesium and chromium compounds for the purposes at hand notonly in isulphuric acid solutions, but in other strong acid solutions,such as hydrochloric, phosphoric, etc., all of which can besatisfactorily used with the same range of concentration (5 to 25 c. c.concentrated acid) as the sulphuric acid with the exception thathydrochlorie acid can be used at concentrations up to 40 c. c. ofconcentrated acid per 100 c. c. of solution.

While we have described our invention in terms of using concentratedacid (96% HzSO4, 36% HC1, 85% H3PO4), it is not limited to such usesince obviously the acid could be diluted with an amount of water equalto that required to produce a solution of the desired strength per 100c. c. without departing from the principle of the invention andaccordingly the claims are to be construed in that light. Our work inconnection with agents to be added to the acid solution indicates thatwhile any suitable magnesium salt can be used, the one having a negativeradicle corresponding to the negative radicle of the acid used, ispreferable. In other words, where sulphuric acid is used, we prefer touse magnesium sulphate as the added magnesium compound.

For simplicity, our present invention will be described in terms oftreating strips of rolled sheet, formed from a magnesium alloyconsisting of approximately'98-5 per cent magnesium and 1.5 per centmanganese. It is to be understood, however, that the invention generallyapplicable to the treatment of magnesium. alloys, and that it is in nosense limited to that specific alloy or article. The aforementionedstrips, before use, were measured for thickness with a micrometer andafter treatment were measured again to determine how much the treatingsolution had reduced the thickness of the strips. As specic examplesshowing the acid treatment and eiect of our agents in reducing theattack of the acid on a magnesium alloy article according to the presentinvention wherein satisfactory cleaning action was obtained, thefollowing are to be noted:

Example 1 Example 2 In similar manner to Example 1, fresh strips wereimmersed for one minute in an aqueous solution containing 7 c. c. ofconcentrated sulphuric acid and 10 grams of magnesium sulphate per 100c. c. of solution. The reduction in thickness per strip was only 0.010of an inch in one minute as compared with the reduction in thickness of0.016 of an inch where the magnesium sulphate was not used.

Example 3 An aqueous solution containing 29 c. c. of concentratedhydrochloric acid (36%) per 100 c. c. of solution, on the averagereduced the thickness of the magnesium alloy strips 0.044 of an inch inone minute while the same strength solution containing 10 grams ofmagnesium chloride reduced the thickness of the strips on the average ofonly 0.029 of an inch.

Example 4 An aqueous solution containing 9 c. c. of phosphoric acid per100 c.,c. of solution, on the average reduced the thickness of themagnesium alloy strips 0.011 of an inch, while the same strengthsolution containing 10 grams of magnesium phosphate reduced thethickness ofthe strips on the average only 0.004 of an inch.

Referring more particularly to the drawing, the curves are based` on.the reduction in thickness of strips of magnesium alloy sheet containing1.5 per cent of manganese, as described in connection with the examplesabove. These curves are based on the average of a number of tests andaccordingly they can be considered as a close approximation of theresults obtainable in the carrying out of the present process. In Fig.2, the curve represents the reduction in thickness of magnesium alloystrips in inches where an aqueous solution containing 14 c. c. ofconcentrated sulphuric acid per c. c. of solution was used, and to whichincreasing amounts of magnesium sulphate up to 40 grams per 100 c. c. ofsolution were added, the sulphuric acid concentration remainingconstant. 'I'he strips, in each case, were immersed in the solution fora period of one minute, following which they were promptly washed,dried, and then measured to determine the reduction in thickness.Referring particularly to the curve, it is to be noted that an aqueoussolution containing 14 c. c. of concentrated sulphuric acid per 100 c.c. of solution reduced the 'thickness of the magnesium-manganese alloystrips approximately 0.027 of an inch in one minute. The use of 20 gramsof magnesium sulphate with 14 c. c. of concentrated sulphuric acid in100 c. c. of aqueous solution decreased the attack so that at the end ofone minute the reduction in thickness'of the strips being treated wasapproximately 0.017 of an inch. In like manner, the use of 40 grams ofmagnesium sulphate and 14 c. c. of concentrated sulphuric acid in 100 c.c. of aqueous solution cut down the attack so that the reduction inthickness of the pieces being treated was approximately 0.001 of aninch. In this connection it is to be noted that even with this highpercentage of magnesium sulphate present in the sulphuric acid solution,the solution still acted in eflicient manner to clean the surface of themetal.

The curves shown on Fig. 1, in similar manner to Fig. 2, are based uponthe treatment of magnesium-manganese alloy strips for one minute in anaqueous sulphuric acid or aqueous sulphuric acid-i-magnesium sulphatesolutions to which increasing amounts of sodium bichromate were added.The use of 40 grams of sodium bichromate with 14 c. c. of concentratedsulphuric acid in 100 c. c. of solution, cuts down the average reductionin thickness of the strips to approximately 0.006 of an inch ascontrasted with the use of an aqueous solution containing 40 grams ofmagnesium sulphate and 14 c. c. of concentrated sulphuric acid per 100c. c. of solution, wherein the reduction of thickness is onlyapproximately 0.001 of an inch. Obviously at the higher concentrations,for instance, 30 to 40 grams of chromium compound or magnesium salt per100 c. c. of acid solution, the use of magnesium sulphate is less costlyand much more eicient than sodium bichromate in reducing the attack ofthe acid on the metal. 'I'he use of 20 grams of magnesium sulphate with14 c. c. of concentrated sulphuric acid in 100 c. c. of aqueous solutionhas the effect of cutting down the reduction in thickness of the strips(one minute treatment) from approximately 0.027 of an inch as noted, toapproximately 0.017 of an inch. The use of sodium bichromate with thissulphuric acid magnesium sulphate solution acts to lessen the reductionin thickness incurred during the treatment up to a point where about 25grams of sodium bichromate are used in 100 c. c. of solution, abovewhich the effect of further additions of sodium bichromate is small anda1- most unnoticeable. 'I'he use of 7 c. c. of concentrated sulphuricacid per 100 c. c. of solution with or without 10 grams of magnesiumsulphate therein acts in somewhat parallel manner to the like solutionscontaining 14 c. c. of concentrated acid with the exception that curvesare pushed to the left to a marked extent, relative to the 14 c. c.sulphuric acid curves, due primarily to the lower concentration of theacid. Looking at the curves and results as a whole, it is to be notedthat the addition of magnesium sulphate to the aqueous sulphuric acidsolution, not only had a marked effect, but the addition of sodiumbichromate to the sulphuric acid-l-magnesium sulphate solution acts tostill further accentuate the protective effect of the magnesium sulphatein the solution. It is thus obvious that the addition of a suitablemagnesium salt, such as magnesium sulphate, to an aqueous strong-acidsolution marks a distinct advantage in cutting down the attack of theacid on the metal and moreover, the addition of suitable chromiumcompounds, such as sodium bichromate, to the strongacid magnesium saltsolution, presents still further advantages for the purpose at hand.

While the curves have been set up to show the use of 7 and 14 c. c. ofconcentrated sulphuric acid per 100 c. c. of solution with or withoutthe addition of other ingredients, the invention is not limited to thosespecific amounts, since obviously the principle holds over a broad rangeof acid concentration, as well as additive ingredients. Where sulphuricacid and a magnesium salt, such as magnesium sulphate, are used, weprefer to use them in an equeous solution, containing per 100 c. c.thereof, sulphuric acid in amount from to 25 c. c. (based onconcentrated acid) and magnesium sulphate in amount from to 40 grams.Specifically a mixture of 10 c. c. of concentrated sulphuric acid with20 grams of magnesium sulphate per 100 c. c. of aqueous solution is welladapted for general use. Where a suitable chromium compound, such assodium bichromate, is used in the above solution in place of themagnesium sulphate, we prefer to use it in amount from 5 to 40 grams per100 c. c. of solution. Specifically, 10 c. c. of cocentrated sulphuricacid and 20 grams of sodium bichromate per 100 c. c. of solution can beused for most purposes. On the other hand, where sulphuric acid,magnesium sulphate, and sodium bichromate are to be used together inaqueous solution, we prefer to use them in the following ranges per 100c. c. of solution: 5 to 25 c. c. of concentrated sulphuric acid, 5 to 30grams of magnesium sulphate, and 5 to 40 grams of sodium bichromate. Aspecific mixture of these materials per 100 c. c. of aqueous solutionthat we have found highly advantageous for use, consists of 10 c. c. ofconcentrated sulphuric acid, 15 grams magnesium sulphate, and 20 gramsof sodium bichromate. The time necessary to leave the magnesium ormagnesium alloy article subjected to, flooded with, or immersed in thesolution depends upon the amount of impurities and surfacecontaminations to be removed, the temperature involved in the treatment,and the strength of the cleaning solution. The examples and curves setforth above are based upon treatment of the magnesium alloy strips atroom temperature or slightly above same, which temperatures we prefer touse although higher temperatures even up to the boiling point of thesolution can be used. It is to be noted in this connection that athigher temperatures the cleaning action of the solution is faster andaccordingly'the time of treatment must be reduced to obtain equivalentVcleaning action to that obtained at room temperature, other factorsremaining constant. Ordinarily, a few seconds up to a minute issuficient for treating the articles in the stronger solutions notedabove, while several minutes may be taken where the articles requireconsiderable cleaning, the temperature of the solution is low, or thesolution is very weak. While we have specifically mentioned sulphuricacid, magnesium sulphate, and sodium bichromate in the examples above,we do not wish to be limited to those exact materials, since, theinvention is applicable to a whole series of equivalent materials ashereinbefore set forth.

For simplicity the term magnesium as used heretofore and in the claimsis to be understood to cover not only magnesium itself, but alloysthereof wherein the magnesium predominates.

Other modes of applying the principle of our invention may be employedinstead of those explained, change being made as regards the process andcomposition herein disclosed, provided the means stated by any of thefollowing claims or the equivalent of such stated means be employed.

We therefore particularly point out and distinctly claim as ourinvention:-

1. A method of treating a magnesium article which comprises subjectingsaid article to the action of an aqueous solution the constituents ofwhich are per 100 cc. thereof, from 5 to 25 cc. of concentratedsulphuric acid, from 5 to 30 grams of a magnesium salt selected from thegroup consisting of magnesium chloride, magnesium sulphate. magnesiumphosphate, and from 5 to 30 grams of a chromium compound selected fromthe group consisting of chromic acid, the chromate and bichromate ofammonia, calcium, magnesium, alkali metals.

2. A method of treating a magnesium article which comprises subjectingsaid article to the action of an aqueous solution the constituents ofwhich are per 100 cc. thereof, from 5 to 25 cc. of concentratedsulphuric acid, from 5 to 30 grams of a magnesium salt selected from thegroup consisting of magnesium chloride, magnesium sulphate, magnesiumphosphate, and from 5 to 30 grams of sodium bichromate. y 3. A method oftreating a magnesium article which comprises subjecting said article tothe action of an aqueous solution the constitutents of which are per 100cc. thereof, from 5 to 25 cc. of concentrated sulphuric acid, from 5 to30 grams of magnesium sulphate, and from 5 to 30 grams of sodiumbichromate.

4. A method of treating a magnesium article which comprises subjectingsaid article to the action of an aqueous solution containing per 100 cc.thereof from 10 to 15 cc. of concentrated sulphuric acid, from 10 to 20grams of magnesium sulphate, and from to 25 grams of sodium bichromate.

5. A method of treating a magnesium article which comprises subjectingsaid article to the action of an aqueous solution the constitutents ofwhich are per hundred cubic centimeters thereof, from 5 to 25 cubiccentimeters of an acid selected from the group consisting of phosphoricacid and sulphuric acid, from 5 to 30 grams of a magnesium salt selectedfrom the group consisting of magnesium chloride, magnesium sulphate, andmagnesium phosphate, and from 5 to 30 grams of a compound selected fromthe group consisting of chromic acid, the chromates and bichromates ofammonia, calcium, magnesium, and alkali metals.

JOHN A. GANN. WILLIAM H. GROSS.

